Generally, optical semiconductor chips such as LEDs (light emitting diode devices), phototransistors, photodiodes, CCDs (charge-coupled devices), and EPROMs (erasable programmable ROMs) are sealed in packages in various forms. Representative examples of the sealing method used in the field of optical semiconductor chips include: (i) a method including mounting an optical semiconductor chip 2 on a ceramic container 1 and hermetically sealing the ceramic container 1 with a transparent glass cover 4 with an adhesive resin 3 therebetween (FIG. 1); (ii) a method including mounting an optical semiconductor chip 2 on a ceramic container 1, filling the ceramic container 1 with a transparent optical semiconductor package sealing resin material 5 composed mainly of a thermosetting epoxy composition, and curing the resin material 5 to hermetically seal the optical semiconductor chip 2 (FIG. 2); and (iii) a method including mounting an optical semiconductor chip 2 on a substrate 6 by wire bonding, potting the optical semiconductor chip 2 with a transparent optical semiconductor package sealing resin material 5, and curing the resin material 5 to hermetically seal the chip 2 (FIG. 3). In recent years, the methods in FIGS. 2 and 3 are mainly used instead of the method in FIG. 1.
The above optical semiconductor package sealing resin material is required to have high thixotropy so that it exhibits low viscosity when discharged from a dispenser during sealing operation, in particular during potting operation, and exhibits high viscosity, i.e., low flowability, after potting. The cured product of the optical semiconductor package sealing resin material is required to have transparency (particularly colorless transparency) that does not vary with time. The difference in linear expansion coefficient between the cured product and a substrate on which an optical semiconductor chip to be sealed is mounted can cause warpage and cracks in the cured product. Therefore, the cured product is required to have properties that can prevent warpage and cracks from occurring, i.e., to have good crack resistance. In addition, an urgent issue is to suppress discoloration of the cured sealing resin material by ultraviolet rays and to suppress deterioration of its mechanical properties.
To solve the above problems, Patent Document 1 proposes that a specific aromatic compound such as a phenol benzoate is added to a curable epoxy composition which constitutes an optical semiconductor package sealing resin material.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-64243